1. Field of the Invention
The present invention relates to the preparation of reproducible artwork and more particularly to an apparatus for modifying images on photographic film.
In the graphic arts industry and the printed circuit industry it is often desirable and/or necessary to modify artwork images on photographic film for purposes that facilitate the production of the finished product and which will presently be explained.
The two similar modifications frequently required are: "adding to or subtracting from" a given amount of all defining edges of the image to be reproduced. Such modifications are commonly referred to as "swells, spreads, or plus images" or "shrinks, chokes, or minus images", respectively.
In graphic arts color printing when images of one color touch images of another color on the final printed sheet, it is required that one image overlap or underlap the other image slightly to compensate for imperfect register of sheets through the printing press. These overlaps or underlaps are obtained by making plus or minus images in the photographic stages before the printing plates are made.
In the printed circuit industry, circuit image bearing films known as "phototools" need to be modified slightly to compensate for inherent gains or inherent losses of image area during the process of printing and etching the circuit board. This requires that the phototool be modified accurately in increments of one thousandth of an inch or less. Another application is that of making a plus image film called a "solder mask" phototool from an original film bearing images of the circuit pads called a "pad master" phototool. The "pad master" when modified for example to a 0.020" plus image, becomes usable for a "solder mask" phototool which is necessary to circuit board production.
2. Description of the Prior Art
The most pertinent prior art is believed to be my U.S. Pat. No. 4,324,489 which generally functions satisfactorily, however, in making micro modifications of film being reproduced some disadvantages have been found. For example, this patent features a pair of wire-like rods capable of flexing and returning to a position of repose which connect a film or copy carriage sheet by a pair of pins to a chassis supported rocker arm wherein an orbital drive guide, also connected by an eccentric pin to the carriage sheet, generates an orbital motion to the carriage sheet relative to an underlying unexposed film. This arrangement requires the copy carriage sheet to be engaged and disengaged each time a film is loaded and exposed which promotes wear and inaccurate fitting of the pin receiving holes in the carriage sheet and the eccentric dial which produces inaccurate results following a setting of predetermined dimension for forming a choke or a spread. Further, the above patent features a register pin equipped registration bar for locating the original film in register with an unexposed film wherein the pin supporting registration bar must be lowered before activating the apparatus for its orbital motion. Failure of the operator to lower the registration bar out of the path of movement of the carriage not only produces unsatisfactory results but could damage the apparatus. Additionally, it was required to form the chassis of heavy weight rigid material, to maintain desirable tolerances during movement of the several components, which is undesirable from an economic standpoint.
The present invention overcomes the above and other disadvantages by hingedly mounting the film carriage sheet on the chassis by zero tolerance hinge pins for vertical pivoting movement toward and away from an underlying film board supported by the chassis and moved in an orbital motion by a motor driven eccentric dial moving an under carriage. The film board being connected with the under carriage and chassis by spherical bearing assemblies. The registration pins are mounted on the film board in forward spaced relation with respect to the film carriage sheet. This arrangement permits permanent engagement of the entire drive train which minimizes wear, provides stationary mounting of register pins for greater accuracy and easier operation, and substantially reduces the negative effect of chassis flex which permits the chassis to be formed from sheet metal. This arrangement greatly increases the accuracy of orbital settings without requiring higher tolerances in eccentric dial mechanism due to a reduction drive ratio provided by the spherical bearing assemblies.